It is known that chromium-based catalyst systems with diphosphine ligands catalyse the selective conversion of ethylene to 1-hexene and/or 1-octene depending on the reaction conditions and choice of ligand structure. In particular, the nature and position of any substituents on the aryl rings connected to the phosphines are crucial influences on the selectivity split between 1-hexene and 1-octene. Of particular interest to industry are catalysts for ethylene tetramerisation, as these catalysts are relatively rare. Octene is a valuable co-monomer for the production of high performance linear low density polyethylenes and elastomers, and few selective on-purpose routes to this chemical are known in industry. By comparison, catalysts for ethylene trimerisation are relatively common, and are used industrially by several companies. By tetramerisation it is meant that at least 30% 1-octene is produced in the process.
Non-limiting examples of selective ethylene tetramerisation catalyst systems include the ubiquitous Cr/bis(phosphino)amine (i.e. ‘PNP’) systems, particularly of the type (Ar1)(Ar2)PN(R)P(Ar3)(Ar4), where Ar1 to Ar4 are aryl groups such as phenyl and R is a hydrocarbyl or a heterohydrocarbyl group, beginning with PNP ligands containing no substituents on the phenyl rings bonded to the P-atoms (e.g. as described in WO 2004/056479) and those with m or p-methoxy groups on the phenyl rings (e.g. as described in WO 2004/056480). In addition to this, PNP systems containing o-fluoro groups on the phenyl rings are described in US 2008/0242811 and US 2010/0081777, and PNP systems bearing pendant donor atoms on the nitrogen linker are described in WO 2007/088329. Multi-site PNP ligands are discussed in US 2008/0027188. In addition to the Cr/PNP systems, chromium systems bearing N,N-bidentate ligands (e.g. as described in US 2006/0247399) can be used. PNP ligands with alkylamine or phosphinoamine groups bonded to one of the PNP phosphines (i.e. ‘PNPNH’ and ‘PNPNP’ ligands) are described in WO 2009/006979. Finally, carbon bridged diphosphine (i.e. ‘PCCP’ ligands) are described in WO 2008/088178 and WO 2009/022770.
Related ethylene trimerisation catalysts with high selectivity for 1-hexene can be obtained by using PNP ligands with ortho-methoxy or ortho-alkyl substituents on the phenyl rings bonded to the P-atoms (e.g. as described in WO2002/04119, WO2004/056477 and WO2010/034101).
When carrying out a process for tetramerisation of ethylene, the aim is to choose a catalyst system and adjust process conditions in order to produce the maximum amount of 1-octene, as opposed to trimerisation processes where catalysts and process conditions are adjusted to produce the maximum amount of 1-hexene. 1-Hexene is also typically co-produced in a tetramerisation process. Consequently, new tetramerisation catalyst systems which increase catalyst selectivity to 1-octene while reducing selectivity to co-products are highly desirable. Alternatively, new tetramerisation catalysts which produce similar amounts of 1-octene to catalysts known in the art, but which produce more 1-hexene (i.e. reduced C4 and C10+ oligomers) would also be desirable.
In several investigations of structure-selectivity relationships for tetramerisation ligands, the effect of various patterns of ortho-substitution on the phenyl rings of the (Ar1)(Ar2)PN(R)P(Ar3)(Ar4) ligand (where Ar1-Ar4 are optionally substituted phenyl groups and R is a hydrocarbyl group) has been studied. For example, the effect of ortho-alkyl groups (Blann et al, Chem. Commun. 2005, 620), ortho-methoxy groups (Overett at all. Chem Commun 2005, 622) and ortho-fluorine groups (US 2010/008177) on selectivity has been reported. These ortho-substitutions may produce significant selectivity benefits in terms of reduced co-products (e.g. C10-C14 secondary products or reduced C6 cyclics). However, in all cases the effect of ortho-substitution is to reduce the 1-octene:1-hexene ratio relative to the equivalent unsubstituted PNP ligand. Consequently, ligand motifs that act to increase the intrinsic 1-octene selectivity and which may be used in combination with a beneficial ortho-substitution motif on the same PNP ligand structure may be particularly beneficial.
The formation of a high molecular weight polymer co-product by the Cr-based ethylene tetramerisation catalyst may present a major technical challenge when commercialising an ethylene tetramerisation process. Polymer fouling of the reactor or downstream sections will reduce plant run time and necessitate shut-downs due to blockages and loss of reaction cooling due to coating of heat exchange surfaces. When running tetramerisation processes at reaction temperatures in the range of 40 to 80° C., as is taught in the art, most of the polymer co-product precipitates in the reactor, which can result in fouling of process equipment. To ensure process reliability and adequate run-times under such reaction conditions, it may be necessary to utilise expensive or energy-intensive process design features.
Running a tetramerisation process at process conditions whereby the polymer co-product remains predominantly dissolved in the liquid reaction medium in the reactor (i.e. a solution phase process) would substantially reduce the possibility of reactor or downstream fouling. In addition, a further benefit of such a process might be that a cheaper or more energy-efficient process design could be used, due to the reduced likelihood of fouling process equipment.
A solution phase process could be achieved by using higher reaction temperatures than typically taught in the art, specifically temperatures of above 80° C. However, the art teaches away from running at higher temperatures due to undesirable effects including poor catalyst activity, increased polymer formation and increased selectivity towards 1-hexene. It is well known in the art of the invention that higher reaction temperatures shift the selectivity from 1-octene towards 1-hexene. New tetramerisation catalysts have been developed that show improved performance at high temperatures, but these modifications reduce the octene:hexene ratio further. In this context, novel tetramerisation catalyst structures that increase the intrinsic selectivity towards 1-octene are highly desirable.